ww.sciencedirect.com

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 6

Available online at w

ScienceDirect

journal homepage: www.elsevier .com/locate/he

A novel hydrogen-sensitive sensor based on Pdnanorings/TNTs composite structure

Xiongbang Wei a,*, Xiaohui Yang a, Tao Wu a, Shuanghong Wu a,Weizhi Li a, Xiaohui Wang a, Zhi Chen a,b

a School of Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu, 610054,

Chinab Department of Electrical & Computer Engineering, Center for Nanoscale Science & Engineering, University of

Kentucky, Lexington, KY, 40506, USA

a r t i c l e i n f o

Article history:

Received 5 June 2017

Received in revised form

19 July 2017

Accepted 20 July 2017

Available online 8 August 2017

Keywords:

Pd nanorings

TiO2 nanotube arrays

Composite structure

Hydrogen sensor

* Corresponding author.E-mail address: weixiongbang@uestc.edu

http://dx.doi.org/10.1016/j.ijhydene.2017.07.10360-3199/© 2017 Hydrogen Energy Publicati

a b s t r a c t

Hydrogen sensors with a novel composite structure comprised of Pd nanorings distributed

on TiO2 nanotube arrays were developed and tested. Effect of the TiO2 nanotube diameter

size, Pd nanorings thickness on the sensors' hydrogen response characteristics were

investigated. Time dependence of resistance of the Pd nanorings/TNTs composite structure

on various hydrogen concentrations was also carried out and demonstrated good room

temperature hydrogen sensitive characteristics. Optimized experiments demonstrated

that the hydrogen sensor composed of 25 nm-thickness Pd nanorings distributed on the

77 nm-diameter size TiO2 nanotube showed a fast response time (3.8 s) and high sensitivity

(92.05%) at 0.8 vol% H2. A hydrogen sensitive characteristics model is proposed and the Pd

nanorings' important role in the hydrogen sensitive mechanisms is described. The

hydrogen sensor's excellent hydrogen sensitive characteristics is ascribed to the Pd

nanorings' quick and continual formation and breakage of multiple passages due to ab-

sorption and desorption of hydrogen atoms.

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

Hydrogen is one of the cleanest and most promising alterna-

tive energy sources. It has been widely used in numerous

fields such as rocket fuels for spacecraft, drop-in hydrogen

fueling station, petroleum, chemical industry, fuel cells, and

biomedical applications [1e10]. However, security issues

should be taken into account because of the low spark ignition

energy and explosive with wide flammable range (4 vol%�75 vol%) of H2 in air [11,12]. Hence, sensors with good sensi-

tivity, fast response and recovery time, long-term stability,

.cn (X. Wei).67ons LLC. Published by Els

and low cost are needed to detect the leakage of hydrogen at

room temperature. In recent years, many groups have inves-

tigated different structures and mechanisms of hydrogen

sensors [13e18]. With emergence of nanotechnology, nano-

structures of functional metal oxide semiconductors (e.g.,

SnO2, ZnO,WO3 and Graphene Oxide, etc.) were used as active

materials of sensors due to their high surface-to-volume ra-

tios [19e24].

Since Gong et al. [25] have successfully synthesized the

highly ordered vertically oriented titania nanotubes (TNTs),

TNTs have attracted significant interest as a sensing material

evier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 6 24581

because of its unique structure and special physical/chemical

properties. In our former work, we have successfully fabri-

cated high-quality self-ordered TiO2 nanotubes on fluorine-

doped tin oxide glass and studied the effects of anodic

oxidation time, F� ion concentration, high temperature

annealing on the preparation of TiO2 nanotube arrays [26].

TNTs fabricated by anodization of titanium offer larger spe-

cific surface area, good stability and favorable electron

mobility [27,28]. In particular, highly ordered and vertically

oriented TNTs, which have controllable pore size, tube length

and facile fabrication process, have made it as a promising

hydrogen sensor material. Varghese et al. [29] have investi-

gated the hydrogen sensing properties of TNTs made via

anodization. The results showed the nanotubes with smaller

pore diameter (46 nm) had greater sensitivity to hydrogen at

290 �C [29]. Sxennik et al. [30] have reported that the sensors

based on TNTs synthesized by anodic oxidation of a titanium

foil in an aqueous solution showed good sensitivity but poor

response/recover time at room temperature. Obviously, these

sensors based on TNTs have excellent hydrogen sensing

properties in a wide range owing to the TNTs' unique nano-

structure. However, the high operating temperature

(200 �Ce500 �C) often limits their applications because

hydrogen is easily explosive at high temperature. In order to

reduce the high operating temperature of TiO2 nanotube

hydrogen sensors, TiO2 nanotubes were often coated with

catalysts (such as Pt or Pd). Aicheng Chen et al. [31] studied the

functionalized TNTs with Pd nanoparticles for hydrogen

sorption and storage, and their studies showed that the TiO2

NT/Pd nanocomposites possess a much higher hydrogen

storage capacity, faster kinetics for hydrogen sorption and

desorption, and higher stability than the nanoporous Pd. Mor

et al. [32] reported TNTs evaporated by a 10 nm-thickness of

Pd film, which exhibits excellent hydrogen sensitivity with a

fully reversible change in electrical resistance of approxi-

mately 175,000% to 1000 ppm H2 at 24 �C. Xiang et al. [33]

fabricated a hydrogen sensor based on TNTs doped with Pd

nanoparticles prepared by reduction of Pd chloride, which is

Fig. 1 e Schematic illustration of the fabrication process of hyd

composite structure.

capable of operating at room temperature with high sensitive

characteristics due to the catalytic effect of Pd nanoparticles.

We report here a novel hydrogen sensor fabricated by

sputtering Pd nanorings on the surface of TiO2 nanotube ar-

rays' tips, which exhibited good hydrogen sensitive charac-

teristics at room temperature. We studied the influence of the

TiO2 nanotube diameter size, Pd nanorings thickness on the

sensors' hydrogen sensitive characteristics. The results

showed that the novel Pd nanorings/TNTs composite struc-

ture has excellent hydrogen sensitive characteristics and

application potential for detection of hydrogen at room

temperature.

Experimental

TNTs arrays were prepared by anodizing a 0.1-mm-thick Ti

foil (99.8% purity) at different anodization voltage using a

standard electrochemical procedure [25,26]. The as-anodized

samples were sonicated in deionized water for a short time

and dried in air. Finally, the anodized samples were annealed

at 500 �C for 4 h in air to obtain crystallized nanotubes. The as-

prepared TiO2 nanotube samples were used as substrates for

fabrication of Pd nanorings/TNTs composite structures,

where Pd nanorings were deposited using DC magnetron

sputtering with a high purity (99.99%) Pd target. Then, the

counter electrodes were prepared by the deposition of two

200-nm-thick silver (Ag) layers using electron beam evapora-

tion. Thus the novel hydrogen-sensitive sensor based on Pd

nanorings/TNTs composite structure were prepared. The

fabrication process of hydrogen sensors in this work is

showed in Fig. 1. Conductive wires (Cu) were connected to the

Ag electrodes with silver conductive paint.

Themorphology of TNTs and the composite structurewere

characterized using scanning electron microscopy (SEM; FEI-

Inspect F50, Holland). The hydrogen sensitive characteristics

measurements were conducted in a gas flow cell made of

Teflon by exposing it at different concentrations of H2 in clean

rogen-sensitive sensor based on Pd nanorings/TNTs

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 624582

air with a constant flow rate of 200 sccm (sccm: standard-state

cubic centimeter per minute). A Keithley 2700 multimeter

(Tektronix China Co., Ltd, Chengdu, China) was used as a data

collector to record the variation of the sensors hydrogen

sensitive characteristics.

Results and discussion

All of the sensors' tests were carried out at room-temperature

(25 �C). The response/recovery time is defined as the time for

the resistance of sensors decreasing/increasing to 90% of the

total change. The hydrogen sensitivity(S) of the Pd nanorings/

TNTs sensor is defined as ratio of sensor resistances in air and

after the hydrogen gas injection, i.e., S ¼ (RA � RH)/RA, where

RA and RH are the resistances values measured in air ambient

and H2 ambient, respectively [34].

For this novel composite structure hydrogen sensor,

diameter size of TiO2 nanotube (D-TNT) has great influence on

the distribution of Pd nanorings, and further on the sensors'hydrogen sensitive characteristics. The D-TNT can be

controlled easily by the anodization voltage in process tech-

nology [25,26,30]. To analyze the influence of the nanotube

size on the sensors' response characteristics, three samples

were prepared at anodization voltages of 20 V, 40 V, and 60 V,

thickness of Pd nanorings were controlled at 30 nm, and

measurements were carried out at 0.5 vol% hydrogen in air.

Table 1 summarizes the response performance of the sensors

based on this structure at different D-TNT. It is clear that the

D-TNT increases with increasing anodization voltage. With in

and off of hydrogen, the electrical resistances of the sensor are

shown in Fig. 2, where RA is the resistance values measured in

air and R is the real time resistance measurement data after

hydrogen injection. It can be calculated that the sensitivity

increases from 5.38% to 54.6% as the D-TNT varies from 28 nm

to 120 nm. For this structural sensor, the dominant hydrogen

sensitive mechanism is due to the combined effect of TNTs

and Pd nanorings. It can be observed that the optimal

hydrogen sensing performance could be achieved as the

sensors fabricated at 40 V, with 90 nm D-TNT. Then, the

sensor's response time is 21 s, and the recovery time is 23 s,

however, the sensitivity is only 24%.

For a certain average D-TNT TiO2 nanotube arrays, the

proper combination of Pd nanorings thickness could show a

better hydrogen sensitive characteristics. In order to study the

effect of Pd nanorings thickness on hydrogen sensing, the

TiO2 nanotube arrays (90 nm D-TNT) covered with 15 nm,

20 nm, 30 nm and 45 nm-thickness Pd nanorings were

deposited on the TNTs. The room-temperature hydrogen

response time and recovery time curves are shown in Fig. 3. As

Fig. 3 showed, the sensor with 30 nm-thickness Pd nanorings

Table 1 e Response performance at 0.5 vol% H2 of thesensors fabricated at different anodization voltages.

Voltage D-TNT Responsetime

Recoverytime

Sensitivity

20 V 28 nm 226 s 56 s 5.38%

40 V 90 nm 21 s 23 s 24%

60 V 120 nm 73.8 s 103.8 s 54.6%

has better hydrogen response time and recovery time. These

curves show that the hydrogen sensing performance of the Pd

nanorings/TNTs composite structure sensors are greatly

influenced by the thickness of Pd nanorings.

Dependence of the Pd nanorings/TNTs relative resistance

(R/RA) on various hydrogen concentrations and a specific

0.5 vol% hydrogen concentration are showed in Fig. 4. The

TNTs (90 nm D-TNT) were covered with 30 nm-thickness Pd

nanorings. As the figures showed, when the test gas flow was

switched to air, the resistance of the sample can well recover

back to its original resistance value quickly. After several tests

for the sensor at various hydrogen concentrations, the similar

hydrogen sensitive behavior was observed.

The overall performance levels of hydrogen sensors based

on this novel Pd nanorings/TNTs composite structure are

influenced by various factors. Important factors include the

geometry andmicrostructures of TiO2 nanotubes and catalytic

additives. For this novel Pd nanorings/TNTs composite

structure, the dominant hydrogen sensitivemechanism is due

to the combined effect of TNTs and Pd nanorings.

On the one hand, when the Pd nanorings/TNTs sensors are

exposed to air, TiO2 first interacts with oxygen atoms adsor-

bed and electrons are transferred from the TiO2 conduction

band to the oxygen atoms, forming O2�, O�, O2� ionic at the

TiO2 interface, due to the TiO2 conduction band minimum is

higher than the chemical potential of O2 [35e37]. Then, when

the sensor is exposed to hydrogen, conductivity of the nano-

composite increases with increasing of hydrogen concentra-

tion due to the catalytic effect of Pd doping [38,39], in this case,

Pd acts as an electron acceptor on semiconducting oxide

surfaces, which contributes to the increase of the depleted

layer. Therefore, the change in resistance is larger as

compared to the pristine oxide case, leading to an enhance-

ment in hydrogen sensing performance. This enhanced gas

sensing performance can be explained by the exchange of

electrons between TiO2 and PdO particles.

Moreover, Pd metals are also known as an excellent

hydrogen sensitive material (about 900 times of its own vol-

ume at room temperature after hydrogen absorption and

Fig. 2 e Response performance curves at 0.5 vol% H2 of the

sensors based on different D-TNT TiO2 nanotube arrays,

with 30 nm-thickness Pd nanorings.

Fig. 3 e The room-temperature hydrogen sensitive characteristics of the sensors with different thicknesses of Pd nanorings

(15 nm, 20 nm, 30 nm, 45 nm). (a) is the response time vs. H2 concentration, and (b) is the recovery time vs. H2 concentration.

Fig. 4 e Room temperature resistance variation of the sensors with 30 nm-thickness Pd nanorings on the surface of TiO2

nanotube arrays for (a) different hydrogen concentrations, and (b) the 0.5 vol% hydrogen concentration.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 6 24583

formed PdHx compound), and Pd has the ability to reversibly

absorb and desorb a large amount of hydrogen [33,40]. When

the sensors are exposed to hydrogen, Pd adsorbs hydrogen

and formed Pd hydride. In this novel Pd nanorings/TNTs

composite structure, the Pd nanorings are separated by the

interstices of TNTs, and the separated Pd nanorings show the

break-junction effect [41] and paly the dominative role on the

hydrogen sensitive characteristics. The space between the

nanorings is different as a result of Pd nanorings thickness. In

order to better understand the response mechanism, SEM

images of the samples and the hydrogen-sensing mechanism

model schematic diagrams are shown in Fig. 5. Fig. 5(a) is SEM

image of a pure TNTs arrays without Pd covered, and Fig. 5(b)

is SEM image of the corresponding Pd nanorings/TNTs com-

posite structure in air. Compared to Fig. 5(a), after deposition

of Pd, certain thickness of Pd nanorings was covered and

separated distributed on the TNTs top surface, as shown in

Fig. 5(b). Fig. 5(c and d) are schematic diagrams of hydrogen

sensing mechanism. In the schematic diagrams, the ringed

distribution of orange solid balls on behalf of Pd nanoparticles,

which formed a series of separated Pd nanorings arrays

because they deposited on the TiO2 nanotube arrays top sur-

face. Fig. 5(c) is the condition of Pd nanorings before absorb H2,

which correspond to the case of Fig. 5(b). Fig. 5(d) is the con-

dition of Pd nanorings after absorb H2. In the proposed model,

the TiO2 nanotube array parameters are assumed to be the

same. Then, as the sensors are exposed to hydrogen, the Pd

nanorings adsorb hydrogen and formed Pd hydride, leading to

a rapid volume expansion. In this case, the expansion of Pd

nanorings causes the separated Pd nanorings to be connected

with each other, and then one or more conducting passage

between the electrodes were created. As a result, the resis-

tance dramatically decreases. In this novel composite struc-

ture, for samples with too thin Pd nanorings, the interval

between the Pd nanorings is relatively large, which results in a

relatively long time to form limited number of conducting

passages as the Pd nanorings exposing to hydrogen. However,

if the Pd nanorings are too thick, continuous Pd nano net-

works have been formed on the surface of TiO2 nanotube ar-

rays before absorb hydrogen. In these two cases, no obvious

break-junction effect could occur. Only when the Pd nanor-

ings have a certain thickness, the separated Pd nanorings

could connect with each other and create more conducting

passages between the electrodes, which results in rapid

resistance decrease. In this case, the ideal isolated Pd nanor-

ings can quickly form or break multiple conducting passages

by absorbing or desorbing hydrogen, and the hydrogen sen-

sors have the optimized response and recovery characteris-

tics, as shown in Fig. 5(d).

Series of process technology were carried out and the

further optimized hydrogen sensitive characteristics of Pd

nanorings/TiO2 nanotubes composite structure sensor at se-

ries of hydrogen concentrations were studied, as shown in

Fig. 6. Diameter size of the optimized TNTs was controlled in

77 nm and the optimized isolated Pd nanorings thickness is

25 nm. From Fig. 6, for the hydrogen concentration ranged

Fig. 5 e SEM images of the samples and the hydrogen-sensing mechanism model schematic diagrams. SEM images of a

pure TNTs arrays (a) and the Pd nanorings/TNTs composite structure in air (b). Schematic diagrams of Pd nanorings

condition before absorb H2 (d), and after absorb H2 (d), and the black line referred to multiple conducting passages formed.

Fig. 6 e Optimized hydrogen sensitive characteristics of the sensor at hydrogen concentration ranged from 0.2 vol% to

1.2 vol%.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 624584

from 0.2 vol% to 1.2 vol%, the response time is 3.8e6 s, and the

sensitivity is also increase with the hydrogen concentration.

As the concentration of hydrogen is 0.8 vol%, the response

time of the sensor is only 3.8 s, and the sensitivity can reach

92.05%. Though the response time and sensitivity of the

hydrogen sensor have been greatly improved, the recovery

time of the hydrogen sensor is still too long as showed in

Table 2. The long recovery time of the hydrogen sensor can be

ascribed to the hydrogen evolution hysteresis effect of Pd

nanorings. In natural state, Pd exists in the form of

Table 2 e Optimized hydrogen sensitive parameter of thesensor at hydrogen concentration ranged from0.2 vol% to1.2 vol%.

H2 Concentration(vol%)

Responsetime (s)

Sensitivity (%) Recoverytime (s)

0.2 3.4 11.47 338

0.4 5.8 45.26 40

0.8 3.8 92.05 43.3

1.2 3.9 92.88 196

elementary substance. After hydrogen absorption, Pd formed

PdHx compound. Because of the chemical bond energy be-

tween palladium and hydrogen, the hydrogen desorption

process of the PdHx nanorings is significantly slower than the

hydrogen absorption process of the Pd nanorings. This

hydrogen sensitive performance is still expected to be

improved by technological process improvement.

Conclusions

Hydrogen sensors fabricated by sputtering Pd nanorings on

TiO2 nanotube arrays were developed and exhibited good

hydrogen sensitive characteristics at room temperature. The

TiO2 nanotube diameter size, Pd nanorings thickness have

great influence on the sensors' hydrogen sensitive perfor-

mance. Optimized experiments demonstrated that the

hydrogen sensor composed of 25 nm-thickness Pd nanorings

distributed on the 77 nm-diameter size TiO2 nanotube showed

a fast response time (3.8 s), and high sensitivity (92.05%) at

0.8 vol% H2. In this novel composite structure, except that the

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 6 24585

TiO2 nanotubes' inherent hydrogen sensitive characteristics

under Pd catalysis, the Pd nanorings “break-junction” effect

play more important role in the hydrogen sensitive mecha-

nisms. A hydrogen sensitive characteristicsmodel is proposed

and the dominant role of Pd nanorings in the hydrogen sen-

sitivemechanisms is described. The hydrogen sensor's perfecthydrogen sensitive characteristics is ascribed to the Pd

nanorings' quick and continual formation and breakage of

multiple conducting passages due to absorption and desorp-

tion of hydrogen atoms.

Acknowledgements

This work was supported by National Natural Science Foun-

dation of China under Grant Nos. 61474016 and 61405026,

61405025, 61371046.

r e f e r e n c e s

[1] Goltsov VA, Veziroglu TN. A step on the road to hydrogencivilization. Int J Hydrogen Energy 2002;27(7e8):719e23.

[2] Cecere D, Giacomazzi E, Ingenito A. A review on hydrogenindustrial aerospace applications. Int J Hydrogen Energy2014;39(20):10731e47.

[3] Agll AA, Hamad TA, Hamad YM, Bapat SG, Sheffield JW.Development of design a drop-in hydrogen fueling station tosupport the early market buildout of hydrogeninfrastructure. Int J Hydrogen Energy 2016;41(10):5284e95.

[4] Reichmuth DS, Lutz AE, Manley DK, Keller JO. Comparison ofthe technical potential for hydrogen, battery electric, andconventional light-duty vehicles to reduce greenhouse gasemissions and petroleum consumption in the United States.Int J Hydrogen Energy 2013;38(2):1200e8.

[5] Xiao RL, Xu SP, Li QX, Su YM. The effects of hydrogen on KOHactivation of petroleum coke. J Anal Appl Pyrol2012;96(12):120e5.

[6] Tonezzer M, Dang TL, Tran QH, Nguyen VH, Iannotta S.Selective hydrogen sensor for liquefied petroleum gas steamreforming fuel cell systems. Int J Hydrogen Energy2017;42(1):740e8.

[7] Tsujimura T, Suzuki Y. The utilization of hydrogen inhydrogen/diesel dual fuel engine. Int J Hydrogen Energy2017;42(19):14019e29.

[8] Cottrell CA, Grasman SE, Thomas M, Matin KB, Sheffield JW.Strategies for stationary and portable fuel cell markets. Int JHydrogen Energy 2011;36(13):7969e75.

[9] Brown LF. A comparative study of fuels for on-boardhydrogen production for fuel-cell-powered automobiles. Int JHydrogen Energy 2001;26(4):381e97.

[10] Ohsawa I, Ishikawa M, Takahashi K, Watanabe M,Nishimaki K, Yamagata K, et al. Hydrogen acts as atherapeutic antioxidant by selectively reducing cytotoxicoxygen radicals. Nat Med 2007;13(6):688e94.

[11] Buttner WJ, Post MB, Burgess R, Rivkin C. An overview ofhydrogen safety sensors and requirements. Int J HydrogenEnergy 2011;36(3):2462e70.

[12] Hubert T, Boon-Brett L, Palmisano V, Bader MA.Developments in gas sensor technology for hydrogen safety.Int J Hydrogen Energy 2014;39(35):20474e83.

[13] Maffei N, Kuriakose AK. A solid-state potentiometric sensorfor hydrogen detection in air. Sens Actuators B2004;98(1):73e6.

[14] Nedrygailov I, Karpov EG. Pd/n-SiC nanofilm sensor formolecular hydrogen detection in oxygen atmosphere. SensActuators B 2010;148(2):388e91.

[15] Debiaggi SRD, Crespo EA, Braschi FU, Bringa EM, Ali ML,Ruda M. Hydrogen absorption in Pd thin-films. Int J HydrogenEnergy 2014;39(16):8590e5.

[16] Gardu~no-Wilches I, Alonso JC. Hydrogen sensors fabricatedwith sprayed NiO, NiO: Li and NiO: Li, Pt thin films. Int JHydrogen Energy 2013;38(10):4213e9.

[17] Kim S, Song Y, Lim HR, Kwon YT, Hwang TY, Song E, et al.Fabrication and characterization of thermochemicalhydrogen sensor with laminated structure. Int J HydrogenEnergy 2017;42(1):749e56.

[18] Chiu SY, Tsai JH, Huang HW, Liang KC, Huang TH, Liu KP,et al. Hydrogen sensors with double dipole layers using a Pd-mixture-Pd triple-layer sensing structure. Sens Actuators B2009;141(2):532e7.

[19] Kadir RA, Li ZY, Sadek AZ, Rani RA, Zoolfakar AS, Field MR,et al. Electrospun granular hollow SnO2 nanofibers hydrogengas sensors operating at low temperatures. J Phys Chem C2014;118(6):3129e39.

[20] Horprathum M, Srichaiyaperk T, Samransuksamer B,Wisitsoraat A, Eiamchai P, Limwichean S. Ultrasensitivehydrogen sensor based on Pt-decorated WO3 nanorodsprepared by glancing-angle dc magnetron sputtering. ACSAppl Mater Interfaces 2014;6(24):22051e60.

[21] Yang XH, Wang W, Xiong JL, Chen L, Ma Y. ZnO: Cd nanorodshydrogen sensor with low operating temperature. Int JHydrogen Energy 2015;40(36):12604e9.

[22] Luo XT, You KK, Hu YM, Yang SL, Pan XM, Wang Z, et al.Rapid hydrogen sensing response and aging of a-MoO3

nanowires paper sensor. Int J Hydrogen Energy2017;42(12):8399e405.

[23] Mondal B, Basumatari B, Das J, Roychaudhury C, Saha H,Mukherjee N. ZnO-SnO2 based composite type gas sensor forselective hydrogen sensing. Sens Actuators B2014;194(194):389e96.

[24] Yang YJ, Li SB, Yang WY, Yuan WT, Xu JH, Jiang YD. In situpolymerization deposition of porous conducting polymer onreduced graphene oxide for gas sensor. ACS Appl MaterInterfaces 2014;6(16):13807e14.

[25] Gong DW, Grimes CA, Singh RS, Arghese OK, Chen Z, Hu WC,et al. Titanium oxide nanotube arrays prepared by anodicoxidation. J Mater Res 2001;169(12):3331e4.

[26] Yang XH, Wei XB, Wu SH, Wu T, Chen Z, Li SB. High-qualityself-ordered TiO2 nanotubes on fluorine-doped tin oxideglass. J Mater Sci Mater Electron 2015;26(9):7081e5.

[27] Hattori M, Noda K, Nishi T, Kobayashi K, Yamada H,Matsushige K. Investigation of electrical transport inanodized single TNTs. Appl Phys Lett 2013;102, 043105.

[28] Yang DJ, Park H, Cho SJ, Kim HG, Choi WY. TiO2-nanotube-based dye-sensitized solar cells fabricated by an efficientanodic oxidation for high surface area. J Phys Chem Solids2008;69(5e6):1272e5.

[29] Varghese OK, Gong D, Paulose M, Ong KG, Grimes CA.Hydrogen sensing using titania nanotubes. Sens Actuators B2003;93(1e3):338e44.

[30] S‚ ennik E, Colak Z, Kılınc N, €Ozturk Z. Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor. Int JHydrogen Energy 2010;35(9):4420e7.

[31] Chen S, Ostrom C, Chen AC. Functionalization of TiO2

nanotubes with palladium nanoparticles for hydrogensorption and storage. Int J Hydrogen Energy2013;38(32):14002e9.

[32] Mor GK, Caralho MA, Varghese OK, Pishko MV, Grimes CA. Aroom-temperature TiO2-nanotube hydrogen sensor able toself-clean photoactively from environmental contamination.J Mater Res 2004;19(2):628e34.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 2 ( 2 0 1 7 ) 2 4 5 8 0e2 4 5 8 624586

[33] Xiang CL, She Z, Zou YJ, Cheng J, Chu HL, Qiu SJ, et al. Aroom-temperature hydrogen sensor based on Pdnanoparticles doped TNTs. Ceram Int 2014;40(10):16343e8.

[34] Kim BJ, Kim JS. Hydrogen sensor using the Pd film supportedon anodic aluminum oxide. Int J Hydrogen Energy2014;39(29):16500e5.

[35] Kwon Y, Kim H, Lee S, Chin IJ, Seong TY, Lee WI, et al.Enhanced ethanol sensing properties of TiO2 nanotubesensors. Sens Actuators B 2012;173(12):441e6.

[36] Luo YF, Zhang C, Zheng B, Geng X, Debliquy M. Hydrogensensors based on noble metal doped metal-oxidesemiconductor: a review. Int J Hydrogen Energy2017;42(31):20386e97.

[37] Woo JA, Phan DT, Jung YW, Jeon KJ. Fast response ofhydrogen sensor using palladium nanocube-TiO2 nanofibercomposites. Int J Hydrogen Energy 2017;42(29):18754e61.

[38] Moon J, Park JA, Lee SJ, Zyung T, Kim ID. Pd-doped TiO2

nanofiber networks for gas sensor applications. SensActuators B 2010;149(1):301e5.

[39] Lee JM, Park J-E, Kim S, Kim S, Lee E, Kim S-J, et al.Ultrasensitive hydrogen gas sensors based on Pd-decoratedtin dioxide nanostructures: room temperature operatingsensors. Int J Hydrogen Energy 2010;35(22):12568e73.

[40] Kolmakov A, Klenov DO, Lilach Y, Stemmer S, Moskovits M.Enhanced gas sensing by individual SnO2 nanowires andnanobelts functionalized with Pd catalyst particles. NanoLett 2005;5(4):667e73.

[41] Wu T, Wei XB, Yang XH, Quan Y, Lv GD, Shi Y, et al. Highlysensitive hydrogen sensor based on Pd-functionalized titaniananotubes prepared in water-contained electrolyte. J MaterSci Mater Electron 2017;28(2):1428e32.